1. Field of the Invention
The present invention relates to a semiconductor light emitting diode and to a method of manufacturing the same.
Priority is claimed on Japanese Patent Application No. 2005-052934, filed Feb. 28, 2005, the contents of which are incorporated herein by reference.
2. Description of Related Art
Semiconductor light emitting diodes that have nitride based compound semiconductors are formed by arranging light emitting portions on a substrate of silicon, ceramic, SiC, or the like. The various types of structure described below may be used for the light emitting portions. Firstly, there is a double hetero structure in which a light emitting layer is sandwiched between a first conductive type of semiconductor layer and a second conductive type of semiconductor layer. Secondly, there is a single quantum well structure (SQW) in which an extremely thin light emitting laser is sandwiched between two semiconductor layers. Thirdly, there is a multiple quantum well structure (MQW) in which the light emitting layer is formed by multiple thin layers. Moreover, in a conventional semiconductor light emitting diode, a first electrode is disposed on the light emitting portion, while, in a conventional semiconductor light emitting diode, a second electrode is disposed on a bottom surface side of a substrate (i.e., on the surface on the opposite side from the light emitting portion side). The second electrode may be connected electrically to the first electrode via the light emitting portion.
In addition, in order to be able to fulfill its role of supporting the light emitting portion, the substrate is formed with a degree of thickness. Therefore, in order to efficiently extract light that is emitted from a light emitting layer, a structure may be considered in which a reflective membrane is provided between the light emitting layer and the substrate. If this type of structure is employed, the light that is emitted from the light emitting layer in the direction of the substrate is reflected by the reflective membrane and discharged to the outside from the first electrode side. Furthermore, a structure may be considered in which the reflectance is improved by superimposing reflective membranes that each have a different refractive index (see Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2003-163368). The reflective membrane described in Patent Document 1 is a multilayer structure formed by a primary reflective membrane that is formed from AlOy, which is a non-conductive material, and a secondary reflective membrane that is formed from AlGaAs that has lower refractive index that that of the primary reflective membrane.
Furthermore, if consideration is given to problems such as the light that is emitted from the light emitting layer by the first electrode becoming attenuated when it is emitted to the outside of the semiconductor device, then if the first electrode is formed not over the entire top surface of the semiconductor layer (i.e., the light emitting portion), but only on a portion thereof, the distance of the current path in each portion between the first electrode and the second electrode is not uniform. Namely, the current path between the first electrode and positions on the second electrode that are directly below the first electrode is shorter than the current path between the first electrode and positions on the second electrode that are not directly below the first electrode. In addition, because a resistance value between the first electrode and second electrode is proportionate to the distance of the current path between the two, current flows in comparatively larger quantity on the current path between the first electrode and the positions on the second electrode that are directly below the first electrode. As a result of this, when current diffusion in a horizontal direction is low, light is emitted only from the vicinity of the light emitting layer directly below the first electrode.
In contrast to this, a structure may be considered in which, by forming the first electrode substantially over the entire top surface of the semiconductor layer (i.e., the light emitting portion), the current can be made to spread to the entire semiconductor light emitting diode, so that the efficiency of the extraction of light from the light emitting layer is improved. Moreover, a structure may also be considered in which the first electrode is formed from a thin electrode material that is optically transparent such that light is not obstructed in the first electrode. Furthermore, a structure may also be considered in which current is made to flow in an entire semiconductor light emitting diode by providing a current blocking layer that is wider than a pad electrode directly below the pad electrode so that the resistance value of the current path directly below the pad electrode is raised.
However, in the conventional semiconductor light emitting diode described in Patent Document 1, because the reflective membrane that is formed from a non-conductive AlOy material is formed around a conductive layer, current is not able to pass through the reflective membrane and must pass through the conductive layer on the inner side of the reflective membrane, resulting in the current path being made smaller. As a result of this, there is a rise in the voltage in the forward direction of the semiconductor light emitting diode.
Furthermore, in the semiconductor light emitting diode described in Patent Document 1, a current obstructing layer is provided directly below the pad electrode that is provided in the center. In a semiconductor light emitting diode that has this type of structure, the current path between the first electrode and the second electrode is lengthened, and the problem rises that there is a rise in the voltage in the forward direction.
Furthermore, in structure of the semiconductor light emitting diode described in Patent Document 1, if a light emitting portion is formed on the reflective membrane, crystal defects tend to occur that are caused by a difference between the size of the lattice of the crystals forming the reflective membrane and the size of the lattice of the crystals forming the light emitting portion.
In contrast to this, a manufacturing method may be considered in which a portion made up of a reflective membrane and a substrate and a portion made up of a light emitting portion are formed in advance, and the two portions are then adhered together. However, in this manufacturing method, problems relating to the adhesion between the two in bond portions and problems relating to the increased complexity of the manufacturing process arise.